NOTATIO

Do we think it's better to beam over the rest here, or not? The original does not -- it's 18th century; but I'm more concerned with playability and legibility than verbatim copying. (I'm beaming the vocal notes anyway!)
OR:
We're in a stately 6/8. It's a hemiola, of course, which might be the factor.

If you consider changing the beaming anyway for better playability: This looks like a hemiola, how about showing this in the beaming? Edit: LilyPond uses a kneed beam by default but that looks weird with the displaced rest IMO. And I just realised that you already said it’s a hemiola.

Nice. I like the idea of showing the hemiola more clearly; though it occurs over a barline elsewhere, and beaming over the bar might be a step too far.

The hemiola beaming seems like a very good idea if you are modernizing the notation anyway. But then the slippery slope thing rears its ugly head if the hemiolas sometimes straddle the bar lines and consistency is desired.

On a different topic, why are the stems so short on the triplets in the first measure of your first example? A Dorico thing? And it puts the triplet numbers so far from the beams.

That's my beam settings, rather than Dorico's default. They probably need adjusting.

In this case I like beaming to the hemiola. But you’re right that it can get out of hand. Definitely don’t beam over the barlines in cases where there are hemiolas over barlines. If they’re all over the place, as a performer I’d rather see “normal” beaming throughout, lest it become confusing or even patronizing to see constant reminders in the beaming that “this Thing here is a hemiola!” And there’s even an interpretive decision there—maybe some of these constantly beamed hemiolas aren’t actually hemiolas, and the engraver has muddied the waters by beaming them that way. But at a major cadence I don’t mind it, and again I think this case does look more attractive as a hemiola.

Just an idea. I wouldn't however rebeam the hemiola.

I second OCTO's suggestion, since I already likes the first example, but the big interval made it look off.

+1